Trenton Forster, who faces first-degree murder and armed criminal action charges in the early morning shooting, was inside a car when he shot Snyder, authorities said. A second officer arrived moments later and fired at Forster, striking him several times and wounding him, they said.

County prosecutor Robert McCulloch, when announcing the charges against Forster on Thursday afternoon, said more charges are "highly likely." He said Forster remained in stable condition and is expected to survive. Forster will be jailed on a $1 million bond once he's released from the hospital.

Police were called shortly after 5 a.m. about a reported disturbance in Green Park, a community of about 2,600 residents in Missouri's south St. Louis County. Belmar said a young man was banging on the door of a house and demanding to be let in. The man had a relationship with a young woman who was inside the home, police spokesman Shawn McGuire said.

Snyder and another officer, whose name hasn't been released, were dispatched in separate cars. Snyder arrived first.

Belmar said Snyder saw the man inside a car and ordered him to show his hands. McCulloch said Forster shot Snyder once with a 9 mm pistol that was found at the scene.

Snyder, who was married and the father of a 2-year-old boy, was pronounced dead at a hospital a short time later.

Forster and the officer who shot him are white, as was Snyder, who had been with the department for four years.

The circumstances that led to the shooting remain under investigation. Belmar said the suspect was the subject of an earlier drug case in which Snyder was an investigator. However, McCulloch said later that Forster was out on bond in relation to a pending felony marijuana-possession case that did not involve Snyder.

Online court records did not show whether Forster has an attorney who could comment on his behalf about Thursday's charges.

Snyder and the other officer were not wearing body cameras, and St. Louis County police vehicles do not have dashboard cameras, Belmar said.

Relations between police and residents in St. Louis County have been closely watched since Michael Brown, a black and unarmed 18-year-old, was fatally shot during an August 2014 confrontation with Darren Wilson, a white Ferguson police officer who was not charged and later resigned.

Green Park is about 24 miles south of Ferguson.

Brown's death led to months of protests that grew violent at times and was a catalyst for the national Black Lives Matter movement and debate over police treatment of minorities. The shooting also prompted a federal investigation into Ferguson's police department and municipal court.

Belmar and other county officers wore black mourning bands over their badges Thursday, and people in the halls of police headquarters in Clayton stopped to console each other. A large flag in a courtyard of the facility was lowered to half-mast.

"It's been a tough day," a somber Belmar said.

Snyder's death "demonstrates the extreme danger that first responders face every day," St. Louis County Executive Steve Stenger said in a statement. "Our police have my steadfast support and I pledge to do everything I can to provide them with all the resources they need to ensure their safety."

It was the first on-duty death involving a St. Louis County officer since Oct. 31, 2000, when Sgt. Richard Weinhold was killed while investigating a disturbance.